One of the key regulators of stromal formation is the platelet-derived growth factor, also called PDGF. PDGF was originally identified as the v-sis oncogene product of the simian sarcoma virus (Heldin, C. H., et al., J Cell Sci Suppl, 1985, 3, 65-76). This growth factor is made up of two peptide chains, referred to as A or B chains which share 60% homology in their primary amino acid sequence. The chains are disulfide cross linked to form the 30 kDa mature protein composed of either AA, BB or AB homo- or heterodimmers. PDGF is found at high levels in platelets, and is expressed by endothelial cells and vascular smooth muscle cells. PDGF binds with high affinity to the PDGF receptor, a 1106 amino acid 124 kDa transmembrane tyrosine kinase receptor (Heldin, C. H., A. Ostman, and L. Ronnstrand, Biochim Biophys Acta, 1998. 1378(1), 79-113). PDGFR is found as homo- or heterodimer chains which have 30% homology overall in their amino acid sequence and 64% homology between their kinase domains (Heldin, C. H., et al. Embo J, 1988, 7(5), 1387-93). PDGFR is a member of a family of tyrosine kinase receptor with split kinase domains that includes VEGFR2 (KDR), c-Kit, and FLT3. The PDGF receptor is expressed primarily on fibroblast, smooth muscle cells, and pericytes and to a lesser extent on neurons, kidney mesangial, Leydig, and Schwann cells of the central nervous system. Upon binding to the receptor, PDGF induces receptor dimerization and undergoes auto- and trans-phosphorylation of tyrosine residues which increase the receptors' kinase activity and promotes the recruitment of downstream effectors through the activation of SH2 protein binding domains. A number of signaling molecules form complexes with activated PDGFR including PI-3-kinase, phospholipase C-gamma, src and GAP (GTPase activating protein for p21-ras) (Soskic, V., et al. Biochemistry, 1999, 38(6), 1757-64). Through the activation of PI-3-kinase, PDGF activates the Rho signaling pathway inducing cell motility and migration, and through the activation of GAP, induces mitogenesis through the activation of p21-ras and the MAPK signaling pathway.
In adults, the major function of PDGF is to facilitate and increase the rate of wound healing and to maintain blood vessel homeostasis (Baker, E. A. and D. J. Leaper, Wound Repair Regen, 2000. 8(5), 392-8; Yu, J., A. Moon, and H. R. Kim, Biochem Biophys Res Commun, 2001. 282(3), 697-700). PDGF is found at high concentrations in platelets and is a potent chemoattractant for fibroblast, smooth muscle cells, neutrophils and macrophages. In addition to its role in wound healing PDGF helps maintain vascular homeostasis. During the development of new blood vessels, PDGF recruits pericytes and smooth muscle cells that are needed for the structural integrity of the vessels. PDGF is thought to play a similar role during tumor neovascularization. As part of its role in angiogenesis, PDGF controls interstitial fluid pressure, regulating the permeability of vessels through its regulation of the interaction between connective tissue cells and the extracellular matrix.
The PDGFR family of ligands is a set of homo- and heterodimeric ligands bound through a disulfide bridge that can be found in three forms, AA, AB and BB. PDGF is a potent mitogen and chemotactic factor for a variety of mesenchymal cells, such as fibroblasts, vascular smooth muscle cells, glomerular mesangial cells and brain glial cells. PDGF has been implicated in a variety of pathological conditions, including cancer, atherosclerosis, restenosis, liver cirrhosis, pulmonary fibrosis, and glomerulonephritis. PDGF exerts its biological activity by binding to the PDGF receptor (PDGFR) inducing receptor dimerization. PDGF-AA induces only α/α receptor dimers, PDGF-AB induces α/α and α/β receptor dimers, and PDGF-BB induces all three receptor dimer combinations. Once dimerized, the PDGFR undergoes trans-phosphorylation on a tyrosine, activating it for intracellular signaling interactions essential that mediate changes in gene expression, cell migration and proliferation.
Following vascular injury the restenotic reparative process is engaged, and within a few days damaged and dying vascular smooth muscle cells (vSMC) release growth factors, such as bFGF, inducing medial vSMC proliferation over the next 3-5 days. The vSMC migrate to the neointima, where approximately half undergo cell cycle proliferation in the intima, and the other half do not divide. PDGF-BB may be a central chemotactic factor involved in wound healing following vascular trauma because it is both mitogenic for cultured vSCM through activation of PDGF receptors, and chemotactic through activation of PDGFRβ. In vivo, PDGF-BB acts predominantly as a chemotactic factor on vSMC. Injection of PDGF-BB has been shown to increase vSMC migration by greater than 10-fold, but proliferation by only 2-fold (A. Jawein et al. J. Clin. Invest. 1992, 89, 507). In addition, anti-PDGF antibodies have been shown to block migration of vSMC, but not their proliferation (G. A. A. Ferns Science 1991, 253, 1129). The PDGFR inhibitor RPR101511A prevented angiographically defined restenosis following angioplasty (G. Bilder et al. Circulation 1999, 99, 3292). Similarly, the PDGFR inhibitor CT52923 was shown to inhibit neointima formation following carotid artery injury in the rat in in vivo studies (J.-C. Yu et al. J. Pharmacol. Exp. Therap. 2001, 298, 1172).
Signal transduction through PDGFR has been linked to vascular smooth muscle cell (vSMC) migration and proliferation leading to allograft vasculopathy and ultimately graft rejection. The PDGFR inhibitor AG-1295 was shown to reduce neointimal formation in aortic allograft vasculopathy in a rat model of neointimal formation (M. Karck et al. Transplantation 2002, 74, 1335).
Despite the biological evidence that PDGFR inhibitors known in the art have the potential to be used in medicines, there remains a need for new inhibitors of this receptor tyrosine kinase.
Diarylureas are a class of serine-threonine kinase inhibitors as well as tyrosine kinase inhibitors well known in the art. The following publications illustrate their utility as an active ingredient in pharmaceutical compositions for the treatment of cancer, angiogenesis disorders, and inflammatory disorders:    Redman et al., Bioorg. Med. Chem. Lett. 2001, 11, 9-12.    Smith et al., Bioorg. Med. Chem. Lett. 2001, 11, 2775-2778.    Dumas et al., Bioorg. Med. Chem. Lett. 2000, 10, 2047-2050.    Dumas et al., Bioorg. Med. Chem. Lett. 2000, 10, 2051-2054.    Ranges et al., Book of Abstracts, 220th ACS National Meeting, Washington, D.C., USA, MEDI 149.    Dumas et al., Bioorg. Med. Chem. Lett. 2002, 12, 1559-1562.    Lowinger et al., Clin. Cancer Res. 2000, 6(suppl.), 335.    Lyons et al., Endocr.-Relat. Cancer 2001, 8, 219-225.    Riedl et al., Book of Abstracts, 92nd AACR Meeting, New Orleans, La., USA, abstract 4956.    Khire et al., Book of Abstracts, 93rd AACR Meeting, San Francisco, Calif., USA, abstract 4211.    Lowinger et al., Curr. Pharm. Design 2002, 8, 99-110.    Regan et al., J. Med. Chem. 2002, 45, 2994-3008.    Pargellis et al., Nature Struct. Biol. 2002, 9(4), 268-272.    Carter et al., Book of Abstracts, 92nd AACR Meeting, New Orleans, La., USA, abstract 4954.    Vincent et al., Book of Abstracts, 38th ASCO Meeting, Orlando, Fla., USA, abstract 1900.    Hilger et al., Book of Abstracts, 38th ASCO Meeting, Orlando, Fla., USA, abstract 1916.    Moore et al., Book of Abstracts, 38th ASCO Meeting, Orlando, Fla., USA, abstract 1816.    Strumberg et al., Book of Abstracts, 38th ASCO Meeting, Orlando, Fla., USA, abstract 121.    Madwed J B: Book of Abstracts, Protein Kinases: Novel Target Identification and Validation for Therapeutic Development, San Diego, Calif., USA, March 2002.    Roberts et al., Book of Abstracts, 38th ASCO Meeting, Orlando, Fla., USA, abstract 473.    Tolcher et al., Book of Abstracts, 38th ASCO Meeting, Orlando, Fla., USA, abstract 334.    Karp et al., Book of Abstracts, 38th AACR Meeting, San Francisco, Calif., USA, abstract 2753.